Question

Just learning Objective-C coming from a Java background. I am trying to write a program that has no purpose other then to teach me how to write functions in this language and I am getting errors everywhere. The problem is since I just started with this language yesterday the errors make no sense to me. Here is what I have so far.

Main Method:

int main (int argc, const char * argv[])
{
    @autoreleasepool
    {
        NSString * prompt = @"Hello World";

        prompt = writePromptMessage(prompt);

        NSLog(@"%@", prompt);
    }

    return 0;
}

Special Method:

NSString *writePromptMessage(NSString * prompt)
{
    return prompt;
}

My Errors:

• Implicit declaration of function writePromptMessage is invalid in C99
• Implicit conversion of ‘int’ to ‘NSString *’ is disallowed with ARC
• Incomplete integer to pointer conversion assigning NSString strong from int
• Conflicting Types for writePromptMessage

Answer 1

Unlike Java, you need to add a function declaration (aka a function prototype) before the point where it’s used. A function declaration is just like the function definition, but without the body (i.e. the code) of the function, and ending with a semicolon:

// This is a function declaration:
NSString *writePromptMessage(NSString * prompt);

// This is a function definition:
NSString *writePromptMessage(NSString * prompt)
{
    // Function body goes here
}

In order to be able to call a function, you need to have written a declaration before the usage site:

NSString *writePromptMessage(NSString * prompt);  // declaration

int main (int argc, const char * argv[])
{
    ...
    writePromptMessage(...);  // usage
}

// definition follows

Alternatively, you can place the whole definition before the usage site, but this won’t always be possible (e.g. if you have two functions which can call each other).

C and Objective-C do actually allow you to call functions that don’t have visible declarations, but this is considered a deprecated feature because it’s easy to misuse and cause subtle errors:

If a function is called without a visible declaration, the compiler creates an implicit
declaration:

• The implicit declaration takes an arbitrary number and types of parameters
• The implicit declaration returns int

So, what happened is that the compiler is assuming that writePromptMessage returns int when it first sees it, which is wrong, and that causes a cascade of other errors. For functions which don’t return int, you must never use implicit function declarations, and for functions which do return int, you should never use implicit function declarations.